A three-kilometre-thick “brown cloud” of man-made pollution, which stretches from the Arabian Peninsula to China to the western Pacific Ocean, is making Asian cities darker, speeding up the melting of Himalayan glaciers and impacting human health, according to a new United Nations Environment Programme (UNEP) report.

Atmospheric Brown Clouds (ABCs), resulting from the burning of fossil fuels and biomass, has resulted in the formation of particles such as black carbon and soot which absorb sunlight and heat the air, experts write in the study released today in Beijing.

The clouds also “mask” the actual warming impact of climate change by anywhere between 20 and 80 per cent because they include sulfates and other chemicals which reflect sunlight and cool the surface.

Shifting weather patternsThe artificial lowering of temperature by ABCs is leading to sharp shifts in weather patterns, causing significant drying in northern China while increasing the risk of flooding in the Asian nation’s south. Monsoon precipitation over India and South-East Asia has dropped up to seven per cent since the 1950s, with the summer monsoon both weakening and shrinking.

Meanwhile, the health and food security of three billion people in Asia are threatened by ABCs, which impacts air quality and agriculture.

Achim Steiner, UNEP’s Executive Director, voiced hope that “Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia” will serve as an early warning of the phenomenon, which he hopes will now be “firmly on the international community’s radar.”

He called on developed countries to help their poorer counterparts attain the technology needed to spur ‘green’ economic growth.

“In doing so, they can not only lift the threat of climate change but also turn off the soot-stream that is feeding the formation of atmospheric brown clouds in many of the world’s regions,” said Steiner.

Livelihoods at stake

The new publication points out 13 megacities as being ABC ‘hotspots’: Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran. Soot levels in these cities comprise 10 per cent of the total mass of all man-made particles.

Since the 1970s, the Chinese city of Guangzhou, among other cities, has witnessed “dimming” – or reduction of sunlight – of more than 20 per cent, it notes.

The solar heating of the atmosphere by ABCs is “suggested to be as important as greenhouse gas warming in accounting for the anomalously large warming trend observed in the elevated regions” such as the Himalayan-Tibetan region, the study says, leading to the retreat of glaciers.

Further, the clouds contain toxic aerosols, carcinogens and other harmful particles, which could result in more people suffering from respiratory disease and cardiovascular problems.

While the effects of the clouds on food production and farmers’ livelihood could be immense, more research must be done to determine their precise role, it acknowledges, adding that the possible impact of ABCs could include elevated levels of ground-level ozone, which could result in massive crop losses of up to 40 per cent in Asia.

Scientists behind the report – produced by UNEP’s Project Atmospheric Brown Cloud – stress that brown clouds also can be found in parts of North America, Europe, Southern Africa and the Amazon Basin, and that they also require urgent and detailed research

Taken from Livescience.comPollution-filled "brown clouds" over the Indian Ocean could warm parts of the lower layers of Earth’s atmosphere as much as greenhouse gases do, a new study finds.

Aerosols, or tiny particles suspended in the air, make up clouds of pollution and are thought to have an overall cooling effect on the atmosphere as they scatter incoming light back out to space. Scientists suspect that this cooling effect could mitigate some of the warming caused by greenhouse gases.

But some aerosols, particularly soot, are very good at absorbing the sun's rays, and this absorption heats the layer of the atmosphere in which they sit.

How the heating from these aerosols compares to that from greenhouse gases has been largely unknown, but this new study, detailed in the Aug. 2 issue of the journal Nature, found that the warming from the two sources is comparable.

Brown clouds During the six-month tropical dry season, soot and other aerosols waft over the Indian Ocean from Asia and form plumes of pollution known as "atmospheric brown clouds".

The researchers used unmanned aerial vehicles (UAVs) to measure the concentration of the aerosols over the ocean and how much heat they absorbed at different levels of the atmosphere.

The first half of the study period saw little pollution and little atmospheric heating, but during the second half, the brown clouds rolled in and atmospheric heating increased by 50 percent.

Using these results, the researchers modeled the heating effects of brown clouds in the region from 1950 to 2000 and found that it was about the same as that caused by greenhouse gases.

Himalayas and megacities

This study shows that the regional influence of aerosols, not just the global average seen in estimates by the Intergovernmental Panel of Climate Change, may be very important, said Peter Pilewskie of the University of Colorado at Boulder, who was not involved with the study.

"And, interestingly, that influence could either enhance (warm, as we've seen in this study) or mitigate (cool) the expected warming by greenhouse gases," Pilewskie said in an email interview.

The overall warming trend caused by greenhouse gases and aerosols in the area have caused the lower atmospheric layer to warm by 0.25 degrees Celsius per decade—twice the rate of warming at the surface, Pilewskie said in an analysis of the study.

This warming has had enormous implications for the Himalayas, where warming has led to a rapid glacier melt.

"The large clouds of pollution in Asia absorb enough solar energy to have climatic impacts," Pilewskie said.

Pilewskie added that other studies are investigating the possible climatic effects of pollution plumes drifting off from megacities (those with 10 million people or more), though he noted that as fewer aerosols are emitted due to the development of cleaner combustion processes, the influence of aerosols is likely to diminish in the future.